Enteric pathogens have been implicated in the pathogenesis of reactive arthritis. The pathophysiology of reactive arthritis is probably multifactorial and involves the nature of the infecting organism and the immunological response of the host. Though a great deal of attention has been paid to the immunological mechanisms and genetic predispositions of the host, little is known about the bacterial determinants that trigger the autoimmune response. Rarely has the actual organism associated with arthritis in a clinical case been isolated and characterized. The goal of this study is to isolate, identify, and characterize enteric bacteria from patients with reactive arthritis. There are two sources of organisms and patients. In a retrospective study, a collection of over 150 frozen cultures of Yersinia enterocolitica maintained at the Children's Hospital will be reviewed. Using both medical records and a survey of the individuals, we will identify cultures that were associated with the development of arthritis. A prospective study will identify new cases of reactive arthritis. All stool and blood cultures positive for enteric pathogens from patients at three hospitals will be collected and stored. Follow-up of these individuals will focus on those patients who develop arthritis and other postinfectious sequelae. Sera from these individuals will be used to characterize antigenic determinants that trigger the autoimmune response. This study will produce a collection of bacterial isolates associated with the development of arthritis. Subsequent studies will be directed at further characterizing the virulence and antigenic properties of these enteric bacteria to gain a better understanding of the underlying pathogenic mechanisms of reactive arthritis.